1. Field of the Invention
The invention relates to the field of magnetic materials and more particularly relates to ferrimagnetic films for microwave signal processing and transmission applications.
2. Background of the Invention
Ferrite single crystals are used in a number of microwave and millimeter-wave devices. Such devices generally operate at, or near, the ferromagnetic resonance frequency. It is important that this frequency be well defined; that is, the resonance should have a low linewidth (low loss) and the center frequency of the resonance should be insensitive to variations in temperature. The usual way to obtain temperature stability is to use a bulk single crystal of ferrite and to fabricate from this crystal a sphere having a well-polished surface.
The method of using a spherical sample to achieve temperature stability can be understood from the following equation for the resonance frequency: EQU .omega..sub.r ={[.gamma.H.sub.o +N.sub.x.sup.a .omega..sub.M +(N.sub.x -N.sub.z).omega..sub.M ][.gamma.H.sub.o +N.sub.y.sup.a .omega..sub.M +(N.sub.y -N.sub.z).omega..sub.M ]}.sup.1/2 ( 1)
("Microwave Ferrites and Ferrimagnetics", B. Lax and K. J. Button, McGraw-Hill (New York) 1962, eq. (4-32)). In this equation .omega..sub.r is the resonance frequency in angular units; .gamma. is the gyromagnetic ratio; H.sub.o is an externally applied d-c magnetic field; N.sub.x, N.sub.y and N.sub.z are the demagnetizing factors determined by the shape of the sample; N.sub.x.sup.a, N.sub.y.sup.a and N.sub.z.sup.a are effective demagnetizing factors which describe the effects of magnetic anisotropy; .omega..sub.M =.gamma.4.pi.M.sub.o where M.sub.o is the magnetic moment per unit volume (or saturation magnetization) of the sample.
In most low-loss ferrites, such as YIG, the major source of temperature instability arises from the .omega..sub.M factors, since the magnetization M.sub.o varies rapidly with temperature. For a spherical sample, N.sub.x =N.sub.y =N.sub.z so that the terms (N.sub.x -N.sub.z).omega..sub.M and (N.sub.y -N.sub.z).omega..sub.M vanish. This removes the major part of the temperature sensitivity, since the anisotropy effects represented by N.sub.x.sup.a .omega..sub.M and N.sub.y.sup.a .omega..sub.M are smaller. These smaller anisotropy effects also can be made to vanish by rotating the sphere so that the applied d-c field H.sub.o lies along an optimum crystallographic direction of the ferrite. This optimum orientation can also compensate for other sources of temperature drift such as the small variation of .gamma. with temperature (R. E. Tokheim and G. F. Johnson, IEEE Trans. Mag., MAG-7 (1971) 267).
The fabrication and alignment of spherical ferrite crystals are tedious and costly processes. Moreover, it is difficult to incorporate spheres into the modern planar microwave and millimeter-wave circuit geometries. Ferrites in the form of single crystal films overcome these drawbacks. Moreover, there is a well-developed technology for producing useful ferrites as single crystal films. Unfortunately films, or discs fabricated from films, do not have a vanishing demagnetizing field contribution. For example, a thin disc which is oriented normal to the applied d-c field (taken to be the Z-axis) has demagnetizing factors N.sub.x =N.sub.y =0 and N.sub.z =1. If the anisotropy is uniaxial along the film normal, then Equation (1) reduces to EQU .omega..sub.r =.gamma.H.sub.o +N.sub.z.sup.a .omega..sub.M -.omega..sub.M ( 2)
or, equivalently, EQU .omega..sub.r /.gamma.=H.sub.o +H.sub.a -4.pi.M.sub.o. (3)
In Equation (3), H.sub.a is the anisotropy field. It represents the effects of anisotropy from all sources and is positive when the anisotropy creates an easy direction of magnetization along the normal to the film. In the case of the bulk crystals which are used for conventional sphere devices, H.sub.a arises from the cubic magnetocrystalline anisotropy. When the ferrite is in the form of a film which is deposited on a substrate, then the substrate may exert a stress on the film. This stress will modify the anisotropy. For example, for the film geometry described above, and assuming a crystallographic orientation with the &lt;111&gt; direction normal to the film, ##EQU1## (P. J. Besser, J. E. Mee, P. E. Elkins, and D. M. Heinz; Mat. Res. Bull. 6 (1971) 1111). In this equation, K.sub.1 is the cubic anisotropy constant which describes the anisotropy effects which would also be found in spheres. The term 3.sigma..lambda..sub.111 /M.sub.o represents the stress-induced anisotropy. .sigma. is the stress which the substrate exerts on the layer and .lambda..sub.111 is the magnetostriction coefficient of the layer. In more general terms, we can write EQU H.sub.a =H.sub.a.sup.K +H.sub.a.sup..sigma. +H.sub.a '. (5)
H.sub.a.sup.K represents the anisotropy arising from the inherent crystal structure. This structure is cubic in the case of YIG; but may have other symmetries for other ferrites. H.sub.a.sup..sigma. represents the anisotropy arising from stress exerted on the ferrite film by the substrate. H.sub.a ' represents other sources of anisotropy such as the so-called "growth induced" effects. The exact mathematical forms of these terms will depend on the crystal structures, crystal orientations and resonance geometries.
3. Prior Art Statement
The most pertinent prior art discovered by applicant relative to this invention is listed herewith.
Zneimer et al, U.S. Pat. No. 3,125,534, issued Mar. 17, 1964, discloses sintered polycrystalline ferrimagnetic garnet wherein low ferromagnetic resonance linewidth garnet such as yttrium iron garnet, lutecium iron garnet, or mixed yttrium-lutecium iron garnet may have its saturation magnetization lowered to a predetermined value and the temperature stability of the saturation magnetization correspondingly improved by the substitution of a predetermined quantity of gadolinium for the yttrium or lutecium. "Such temperature stability is obtained at some expense inasmuch as the line width increases as the gadolinium content is increased." (Column 6, lines 65-67.) Zneimer et al also suggests that the same effect may be obtained in an yttrium iron garnet having some aluminum partially substituted for iron.
Harrison et al, U.S. Pat. No. 3,132,105, issued May 5, 1964, discloses sintered polycrystalline ferrimagnetic garnet materials such as yttrium gadolinium iron garnet wherein varying amounts of aluminum and gallium are partially substituted for iron, varying amounts of dysprosium are partially substituted for yttrium, and varying amounts of gadolinium are partially substituted for yttrium to reduce the saturation magnetization and correspondingly increase the temperature stability of the saturation magnetization of the materials.
Schieber, U.S. Pat. No. 3,193,502, issued July 6, 1965, discloses ternary ferrimagnetic compositions of matter comprising either iron together with one element of Group III-B of the Periodic System (which includes lanthanum) and one element of the group strontium, barium, calcium and lead or iron together with two elements of Group III-B of the Periodic System either in combination with oxygen alone, or in combination with oxygen and fluorine.
Linares, U.S. Pat. No. 3,486,937, discloses the tipping method of liquid phase epitaxy (LPE) wherein monocrystalline thin films of ferrimagnetic materials are grown on single crystal substrates from fluxed melts. The ferrimagnetic materials are ferrimagnetic garnets, ferrimagnetic spinels, and ferrimagnetic hexagonals. For the garnets, the iron may be mixed with aluminum, gallium, scandium, chromium, or cobalt.
Le Craw, U.S. Pat. No. 3,495,189, issued Feb. 10, 1970, discloses bulk single-crystal iron-containing ferrimagnetic garnet having selected nonmagnetic ions, notably gallium and aluminum but also vanadium, substituted for some of the iron therein primarily on the tetrahedral sites of the crystal. Le Craw teaches that the effect of such substitution is to decrease the saturation magnetization of the garnet to a desired lower value. Le Craw further teaches that substitution of the selected nonmagnetic ions reduces the Curie (or Neel) temperature and that substitutions beyond a certain amount result in increasing temperature sensitivity and are particularly undesirable for room temperature operation (Col. 4, lines 31-42). In addition, Le Craw teaches that, in general, the very large class of rare-earth iron garnets, except for yttrium iron garnet (YIG) and lutecium iron garnet, have significant loss mechanisms associated with their cations.
Kolb et al, U.S. Pat. No. 3,496,108, issued Feb. 17, 1970, discloses a hydrothermal method for growing bulk single-crystal ferrimagnetic garnet such as YIG and partially substituted YIG wherein at least one of the trivalent rare-earth elements, including lanthanum, may be partially substituted for the yttrium and wherein gallium and/or aluminum may be partially substituted for the iron.
Heinz, U.S. Pat. No. 3,995,093, issued Nov. 30, 1976, discloses a garnet bubble domain material for high frequency operation exhibiting a relatively high uniaxial anisotropy having contributions from cubic, stress-induced, and growth-induced anisotropy effects. Heinz teaches that the magnitude of the stress-induced effect is generally limited because stress must be kept small enough that film cracking does not result. Heinz teaches making the growth-induced effect relatively large to produce high uniaxial anisotropy. The preferred material includes both lanthanum and lutecium on the dodecahedral lattice sites and a nonmagnetic ion having a charge of +3, preferably gallium, and iron on the tetrahedral lattice sites. The non-magnetic ion reduces the saturation magnetization of the material. In a more general case, where iron substitution is achieved with an ion having a charge of greater than +3 on tetrahedral lattice sites, a charge compensating ion having a charge of +1 or +2 is also substituted on the dodecahedral lattice sites. The lanthanum and lutecium are preferred, in part because they are non-magnetic. The relative proportions of the various ions are chosen to produce a selected saturation magnetization and a small misfit, or mismatch, between the lattice constant of the bubble domain film and the lattice constant of the single crystal substrate.
Hoekstra et al, "The Origin of Uniaxial Anisotropy in Thin Films of (YLaPb).sub.3 (FeGa).sub.5 O.sub.12 and its Variation Along the Growth Direction", Mat. Res. Bull., Vol. 12, pp. 53-64, 1977, discloses that the uniaxial anisotropy of films of YIG doped with lanthanum and gallium and grown at temperatures above 870.degree. C. is primarily the result of stress-induced anisotropy. The relatively high growth temperature is used to eliminate growth-induced anisotropy resulting from the incorporation of lead from lead-oxide based flux in the films. The compositions disclosed have the general formula Y.sub.3-x-z La.sub.x Pb.sub.z Fe.sub.5-y Ga.sub.y O.sub.12 where x is about 0.2 atoms of lanthanum per formula unit, z is from zero to 0.1 atoms of lead per formula unit, and y is about 1.25 atoms of gallium per formula unit.